To grind a simple lens, mount the blank on a spindle with pitch and grind the surface against a metal tool carrying the inverse curve, moving through silicon carbide of 80, 220, 600 and 1200 grit, then polish with cerium oxide on a pitch lap. A smooth, uniform reflection indicates a good surface. A 50 mm lens ground to a 200 mm radius needs only 1.57 mm of glass removed at the centre, so the blank rarely needs to be thick; the difficulty is keeping the curve spherical, not removing material.
Part of Glass Blowing and Forming
The grinding and polishing sequence that turns a pressed blank into a convex or concave lens.
Grinding Is Shallow, So The Skill Is In Keeping The Curve Even
The carrier gives the sequence; the corpus’s Lens Grinding article gives the fuller method (three stages, constant rotation of the blank, a pine-pitch lap). This article adds the depth arithmetic and the dust hazard.
| Stage | Abrasive (carrier) | Purpose |
|---|---|---|
| Rough | Silicon carbide 80 | Establish the curve |
| Medium | 220 | Refine it |
| Fine | 600 | Smooth it |
| Finest | 1200 | Remove the last scratches |
| Polish | Cerium oxide on a pitch lap | Transparency |
The lens-grinding article uses iron oxide and cerium oxide for the polish; the lenses article uses rouge or iron oxide. Cerium oxide is rare, so rouge made by heating iron filings in air is the usual substitute.
What You Need
- A blank, pressed per Pressing Lens Blanks.
- A tool with the inverse curve: for a convex lens, a concave tool.
- Abrasives in the grit sequence above, each kept in its own covered pot.
- Pitch for mounting the blank and for the lap; see Pitch Making.
- A spindle or hand-rotated mount, and plenty of water.
Method: Grinding And Polishing
Step 1: Mount the blank on the spindle with pitch.
Step 2: Grind with the first grit, with water, using strokes that vary and a regular turn of the blank, until the curve matches the tool.
Step 3: Clean everything, then change grit. One coarse grain left in a finer stage scratches deeply, a mistake the lens-grinding article warns of.
Step 4: Grind through each grit in turn, 80, 220, 600, 1200.
Step 5: Polish on the pitch lap.
Step 6: Test by reflected light. A smooth, uniform reflection indicates a good surface.
Worked Example
How much glass has to go? A spherical surface of radius R across a lens of radius r has a sag s = R − √(R² − r²).
For R = 200 mm and a 50 mm lens (r = 25 mm): R² = 40,000, r² = 625, R² − r² = 39,375, √39,375 ≈ 198.43, so s = 200 − 198.43 = 1.57 mm at the centre relative to the edge. A blank 15-20 mm thick therefore has far more glass than the curve needs. The focal length is R ÷ (n − 1) = 200 ÷ 0.5 = 400 mm, matching the Lens Grinding article’s figure.
Common Mistakes
- Skipping a grit or leaving coarse grit about. Scratches stay.
- Grinding in one fixed direction. The surface goes non-spherical.
- Letting the slurry dry. It becomes dust.
Safety
Silica dust from grinding
Dry slurry from sand or glass grinding becomes fine dust, and crystalline silica dust scars the lung permanently. Keep the work wet at all times, wipe up spills with a wet cloth, never sweep dry and wear the best respirator you have. See Dust and Respiratory Protection. Silicon carbide dust deserves the same care.
Hot pitch
Pitch is melted to mount blanks and make laps. Hot pitch burns and sticks to skin, and its fumes are flammable. Melt it away from open flame in a small vessel and keep water out of the pot.
What’s Next
- Learn next: Lens Grinding — the fuller method
- Combine with: Magnifying Glass — a first lens to make